In the NW of Chihuahua City, Mexico, San Marcos range formation is located. San Marcos area includes two uranium mineral outcrops, a river and its dam, which are the main water supply for the nearest agricultural lands. Assessing fractionation and distribution of U and Th isotopes contained in colloidal and dis-solved fractions of surface water is the aim of this work. U and Th isotopes’ activities were obtained by alpha spectrometry. 238U and 232Th contents in colloids ranged from 5 to 16, and from 0.3 to 0.5 mBq·g
, respectively, with activity ratio (AR, 238U/234U) of up 6. The U distribution coefficient (kdU) ranged from 1·10
. The results have shown a possible lixiviation of uranium from geo-logical substrate into the surface water and an important fractionation of uranium and thorium.